Recently, as the portability and cordless tendency of instruments have progressed, a demand for a non-aqueous electrolyte secondary battery such as a lithium secondary battery which is small in size and light in weight and has a high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal or the like (which may be referred to as a “lithium-containing composite oxide”) such as LiCoO2, LiNi1/3CO1/3Mn1/3O2, LiNiO2, LiNi0.8CO0.2O2, LiMn2O4 or LiMnO2, has been known.
Among others, a lithium secondary battery using LiCoO2 as a cathode active material and using a lithium alloy or carbon such as graphite or carbon fiber as a negative electrode, can obtain a high voltage at a level of 4 V, whereby it has been widely used as a battery having a high energy density.
However, in the case of the non-aqueous type secondary battery using LiCoO2 as a cathode active material, further improvement has been desired e.g. in the discharge capacity, in the stability against heat during heating (which may be referred to as a “safety” in the present invention) and in the capacity density per unit volume of the positive electrode layer (which may be referred to as a “volume capacity density” in the present invention), and it had a problem of e.g. deterioration in the durability for charge and discharge cycles such that the discharge capacity of the battery gradually decreases by repetitive charge and discharge cycles.
In order to solve these problems, various studies have been made heretofore. For example, it has been proposed to prepare a surface modified lithium composite oxide, by dispersing a mixture obtained by mixing a preliminarily synthesized lithium-containing composite oxide, titanium hydroxide and lithium hydroxide, into ethanol, and further wet-mixing it in a ball mill, followed by heat treatment, so that a lithium titanium composite oxide be present at the surface of particles (cf. Patent Document 1). Further, it has been proposed to prepare a surface modified lithium composite oxide, by subjecting a preliminarily synthesized lithium-containing composite oxide and a preliminarily synthesized lithium titanium composite oxide to particle composite treatment by using a mechanofusion device, or to coating treatment by using a high speed mixing flowage machine, so that a lithium titanium composite oxide be present at the surface of the particles (cf. Patent Documents 2 and 3).
Further, it has been proposed to prepare a surface modified lithium composite oxide, by adding a lithium source to a coprecipitated product having titanium sulfate and cobalt sulfate coprecipitated, followed by firing, so that a titanium compound be present at the surface of particles (cf. Patent Document 4). Furthermore, it has been proposed to prepare a surface modified lithium composite oxide, by adding lithium hydroxide and titanium tetrachloride to a liquid having a preliminarily synthesized lithium-containing composite oxide dispersed, followed by heat treatment, so that a lithium titanium composite oxide be present at the surface of particles (cf. Patent Document 5).
Patent Document 1: JP-A-2006-202702
Patent Document 2: JP-A-2004-319105
Patent Document 3: JP-A-2004-103556
Patent Document 4: JP-A-2005-123111
Patent Document 5: JP-A-2002-151078